﻿using System;
using System.Collections;
using System.Collections.Generic;
using System.Data.Common;
using System.Reflection;
using System.Text;

namespace ConsoleApp.MyLinq
{
    internal class ObjectReader<T> : IEnumerable<T>, IEnumerable where T : class, new()
    {
        Enumerator enumerator;
        internal ObjectReader(DbDataReader reader)
        {
            this.enumerator = new Enumerator(reader);
        }

        public IEnumerator<T> GetEnumerator()
        {
            Enumerator e = this.enumerator;
            if (e == null)
            {
                throw new InvalidOperationException("Cannot enumerate more than once");
            }
            this.enumerator = null;
            return e;
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return this.GetEnumerator();
        }

        class Enumerator : IEnumerator<T>, IEnumerator, IDisposable
        {
            DbDataReader reader;
            FieldInfo[] fields;
            int[] fieldLookup;
            T current;
            internal Enumerator(DbDataReader reader)
            {

                this.reader = reader;
                this.fields = typeof(T).GetFields();
            }

            public T Current
            {
                get { return this.current; }
            }

            object IEnumerator.Current
            {
                get { return this.current; }
            }

            public bool MoveNext()
            {
                if (this.reader.Read())
                {
                    if (this.fieldLookup == null)
                    {
                        this.InitFieldLookup();
                    }
                    T instance = new T();
                    for (int i = 0, n = this.fields.Length; i < n; i++)
                    {
                        int index = this.fieldLookup[i];
                        if (index >= 0)
                        {
                            FieldInfo fi = this.fields[i];
                            if (this.reader.IsDBNull(index))
                            {
                                fi.SetValue(instance, null);
                            }
                            else
                            {
                                fi.SetValue(instance, this.reader.GetValue(index));
                            }
                        }
                    }
                    this.current = instance;
                    return true;
                }
                return false;
            }

            public void Reset()
            {
            }

            public void Dispose()
            {
                this.reader.Dispose();
            }

            private void InitFieldLookup()
            {
                Dictionary<string, int> map = new Dictionary<string, int>(StringComparer.InvariantCultureIgnoreCase);
                for (int i = 0, n = this.reader.FieldCount; i < n; i++)
                {
                    map.Add(this.reader.GetName(i), i);
                }

                this.fieldLookup = new int[this.fields.Length];
                for (int i = 0, n = this.fields.Length; i < n; i++)
                {
                    int index;
                    if (map.TryGetValue(this.fields[i].Name, out index))
                    {
                        this.fieldLookup[i] = index;
                    }
                    else
                    {
                        this.fieldLookup[i] = -1;
                    }
                }
            }
        }
    }
}
